1. Field of the Invention
The present invention relates to fluid-operated brake systems of the type including service and emergency brake actuators, and more particularly to a brake system including a valve for effecting a controlled application of the emergency brake actuators in the event of a failure in the service brake control system.
2. Prior Art
Tractor-trailer brake systems are known which include trailer brakes having tandem service and emergency brake chambers. Separate service and emergency brake conduits communicate respectively with the service and emergency brake chambers.
Service brake application or engagement is effected by supplying pressurized fluid to the service brake chambers through the service brake conduit. Service brake disengagement is effected by venting the service brake conduit to atmosphere to exhaust the service brake chambers. The force of service brake application is proportional to the pressure of the fluid in the service brake chambers.
Emergency brake application or engagement is effected by exhausting the emergency brake chambers to atmosphere through the emergency brake conduit to permit braking members to be spring biased into engagement. Emergency brake disengagement is effected by supplying pressurized fluid through the emergency brake conduit to the emergency brake chambers to overcome the spring bias and disengage the braking members.
The emergency brakes are normally held off, i.e., disengaged, when the vehicle is in operation. When the movement of the vehicle is to be braked, pressurized fluid is supplied to the service brake chambers by a control system which communicates with the service brake conduit. When the vehicle is to be parked, the emergency brake conduit is vented to atmosphere, bringing the emergency brakes into full braking engagement.
Proposals have been made in an effort to bring the emergency brake system into operation if a failure should occur in the service brake system. A problem with most such proposals has been that if the vehicle is moving when the emergency brakes are brought rapidly and fully into play, the emergency-braked wheels may skid with a resulting loss in vehicle control.
A number of proposals have been made in an effort to provide controlled, modulated operation of the emergency brakes in the event of a failure in the service brake system. Such proposals have resulted in complex and expensive multi-valve systems that are sluggish in responding to a control signal, lacking in reliability, and do not effect a desirably smooth, controlled application of the emergency brakes.